1. Technical Field
The present disclosure relates to fatigue damage resistant wire and, in particular, relates to a method of manufacturing wire made of a shape memory alloy, which demonstrates improved fatigue strength properties, as well as medical devices made with such wire.
2. Description of the Related Art
Shape memory materials are materials that “remember” their original shape, and which, after being deformed, return to that shape either spontaneously or by applying heat to raise their temperature above a processing and material related threshold known as the transformation temperature. Heating to recover shape is commonly referred to in the art as “shape memory”, whereas spontaneous recovery is commonly referred to as pseudoelasticity. Pseudoelasticity, sometimes called superelasticity, is a reversible response to an applied stress, caused by a phase transformation between the austenite or parent phase and the martensite or daughter phase of a crystal. It is exhibited in shape memory alloys. Pseudoelasticity and shape memory both arise from the reversible motion of domain boundaries during the phase transformation, rather than just bond stretching or the introduction of defects in the crystal lattice. A pseudoelastic material may return to its previous shape after the removal of even relatively high applied strains by heating. For example, even if the secondary or daughter domain boundaries do become pinned, for example due to dislocations associated with plasticity, they may be reverted to the primary or parent phase by stresses generated through heating. Examples of shape memory materials include iron-chrome-nickel, iron-manganese, iron-palladium, iron-platinum, iron-nickel-cobalt-titanium, iron-nickel-cobalt-tantalum-aluminum-boron, copper-zinc-aluminum, copper-zinc-aluminum-nickel, copper-aluminum-nickel, and nickel-titanium alloys. Shape memory materials can also be alloyed with other materials including zinc, copper, gold, and iron.
Shape memory materials are presently used in a variety of applications. For example, a variety of military, medical, safety and robotic applications for shape memory materials are known. Medical grade shape memory materials are used for orthodontic wires, guide wires to guide catheters through blood vessels, surgical anchoring devices and stent applications, for example. One shape memory material in wide use, particularly in medical device applications, is a nickel-titanium shape memory material known as “Nitinol”.
Many medical grade shape memory wire products are made of biocompatible implant grade materials including “NiTi” materials. As used herein, “nickel-titanium material”, “nickel-titanium shape memory material” and “NiTi” refer to the family of nickel-titanium shape memory materials including Nitinol (an approximately equiatomic nickel-titanium, binary shape memory material) as well as alloys including nickel and titanium as primary constituents but which also include one or more additional elements as secondary constituents, such as Nitinol tertiary or quaternary alloys (Nitinol with additive metals such as chromium, tantalum, palladium, platinum, iron, cobalt, tungsten, iridium and gold).
Significant research has been dedicated to understanding how NiTi behaves in the body from the viewpoint of biological host response, but much less has been published that quantitatively correlates structure with mechanical properties.
More particularly, the fatigue properties of NiTi material have been the subject of recent research. The fatigue crack propagation behavior of Nitinol was studied in detail by McKelvey and Ritchie, as published in Fatigue-Crack Growth Behavior in the Superelastic and Shape-Memory Alloy Nitinol, Metallurgical and Materials Transactions, 32A, 2001, pgs. 731-743. McKelvey et al. observed that the crack growth propagation rate and ΔKth, which denotes the stress-intensity fatigue threshold in a given fatigue-crack growth scenario, were different for equivalent composition at martensite-stable and austenite-stable temperatures where the crack growth rate was generally lower at martensite-stable temperatures. They also observed that, under plane strain conditions, the heavily slipped material near the crack tip at superelastic regime temperatures remained austenitic, presumably inhibited from undergoing volume contractile, stress-induced phase transformation by the triaxial stress state, while plane stress conditions generally resulted in stress-induced martensite near the crack tip.
Wire products made of shape memory materials are manufactured by forming a relatively thick piece of hot-worked rod stock from a melt process. The rod stock is then further processed into wires by drawing the rod stock down to a thin diameter wire. During a drawing process, often referred to as a “cold working” process, a wire is pulled through a lubricated die to reduce its diameter. The deformation associated with wire drawing increases the stress in the material, and the stress eventually must be relieved by various methods of heat treatment or annealing at elevated temperatures to restore ductility, thus enabling the material to be further cold worked to a smaller diameter. Conventional wire annealing typically results in grain growth with a concomitant random crystal orientation, and the various material or fiber “textures” that are generated during cold wire drawing are mostly eliminated during conventional annealing and recrystallization. These iterative processes of cold working and annealing may be repeated several times before a wire of a desired diameter is produced and processing is completed.
Wire materials manufactured by the above processes typically contain microstructural defects, such as pores, inclusions, interstitials, and dislocations. An inclusion comprises a phase which possesses distinct properties from the primary material matrix and is divided from the matrix by a phase boundary. Inclusions may result from oxide or other metallic or non-metallic precipitate formation during primary melting or other high temperature treatment and may include carbides, nitrides, silicides, oxides or other types of particles. Inclusions may also arise from contamination of the primary melt materials or from the mold which contains the molten ingot. In the case of an interstitial, an atom occupies a site in the crystal structure at which there usually is not an atom. The atom may be a part of its host material, such as a base metal or alloying metal, or it may be an impurity. A dislocation is a linear defect around which some of the atoms of the crystal lattice are misaligned and appear as either edge dislocations or screw dislocations. Edge dislocations are caused by the termination of a plane of atoms in the middle of a crystal, while a screw dislocation comprises an internal structure in which a helical path is traced around the linear defect or dislocation line by the atomic planes of atoms in the crystal lattice. Mixed dislocations, combining aspects of screw and edge dislocations, may also occur.
Internal or external defects, such as inclusions, pores, or defects induced during wire processing may weaken the host material at the site of the defect, potentially resulting in failure of a material at the site of that defect. This weakening may be particularly acute where the defect is relatively large and/or of significantly disparate stiffness compared with adjacent dimensions of the material (such as for fine or small diameter wire). Failure of shape memory wires is more likely to occur at the site of the defect. Since inherent defects cannot be completely eliminated from the wire material, management of inherent defects and mitigation of their negative impact on wire properties is desirable.
One previously proposed solution to the problem of inherent defects has been to treat selected regions of a wire that are expected to be subjected to high strain by converting the bulk material in such regions to a different phase than the remainder of the bulk material of the wire. For example, under predetermined operating conditions, such as a predetermined operation temperature, the high strain wire regions are stabilized in a martensite phase while the lesser strain regions remain in an austenite phase. This method is therefore directed to treating predetermined regions of a wire to convert the bulk material in the regions to a more stable phase regardless of the presence, number, and location of any defects in the bulk material.
However, it may not always be possible or practical to predict what regions of a continuous wire will be subjected to high strains when portions of the wire are later incorporated into a medical device. It may also be desirable to leave defect-free portions of wire unaffected by mitigation efforts and, therefore, available to meet other design considerations. For example, a disadvantage of the above process is that for wire made of shape memory material, the regions that are stabilized in the martensite phase will lose the superelastic characteristic.
Although wires made in accordance with foregoing processes may demonstrate excellent fatigue strength, further improvements in fatigue strength are desired, particularly with reference to fatigue damage that propagates from defects.
What is needed is a method of manufacturing a wire that demonstrates improved fatigue strength, and medical devices that include such wire.